The aim of this project is to determine the molecular mechanisms causing the inefficiency with which heterospecific DNA is integrated into the genome of a recipient bacterium as well as the relationships between these mechanisms and those causing the relatively low transforming activity of certain markers in homospecific DNA. The rationale is to examine the fate of heterospecific DNA and of low efficiency markers in homospecific DNA after they have been irreversibly absorbed by recipient bacteria. The plan is to compare the changes in physical and/or genetic properties of such DNA with the changes known to occur with high efficiency markers in homospecific DNA. Specific differences in the manner in which heterospecific DNA is handled are expected to suggest a model for the mechanism of discrimination in integration of absorbed donor markers, a model to be tested by its ability to predict the alterations caused by physiological agents and mutations affecting discrimination.